You don't, you take it to your local jeweller who will remove the back and put in a new battery for between $8 and $15 depending on the type of battery, silver oxide or lithium. All done whilst you wait or shop.

Moses, Get all the info. and instructions for Nautica watch at the links below.
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."http://nauticawatches.com/men